It is our objective to carry out detailed structure analyses of enzymes by the established techniques of x-ray crystallography. We propose to continue studies of the flavoprotein enzymes involved in biological oxidations, already initiated by the determination of the crystal structures of all three oxidation states of clostridial flavodoxin. Our choice of enzymes appropriate for structure analysis has been determined by the desire to 1) elucidate enzyme-substrate interactions, 2) understand the changes of structure accompanying oxidation or reduction of the prosthetic group, 3) correlate the structure with biological reactivity and physical properties. The results obtained with flavodoxin from Clostridium MP have led to hypotheses concerning the ways in which a protein structure can modulate the oxidation-reduction potentials of bound prosthetic groups. Comparative studies of another flavodoxin, from A. nidulans, will test these theories. Conformations of the known flavodoxins, derived from crystallographic refinement, are being ultilized to simplify solution of this new structure. Determinations of the three-dimensional structures of the flavoprotein oxidoreductase, ferredoxin-NADP-reductase (FNR) and of its molecular complex with the protein substrate, ferredoxin, are proposed. Ferredoxin-NADP-reductase has been crystallized in this and other laboratories; we have also co-crystalized the reductase with a 2Fe/2S ferredoxin. Spectral and functional properties of FNR ferredoxin complexes will be examined both in solution and in the crystalline state. Two other structure analyses of enzymes catalyzing biological oxidations are in progress: a study of cytochrome c554, a photosynthetic electron carrier from the blue green alga, Anacystis nidulans, and a determination of the structure of iron superoxide dismutase from E. coli.